Non-volatile memory devices such as magneticrandom access memory (MRAM) devices are of interest for replacement of volatile memory devices such as dynamic random access memory (DRAM) devices. Such MRAM devices include an array of individual MRAM cells which may be tunnelling magnetoresistance memory (TMR) cells, colossal magnetoresistance memory cells (CMR) or giant magnetoresistance memory (GMR) cells.
In general, the MRAM cells include a data layer and a reference layer. The data layer is composed of a magnetic material and during a write operation the magnetization of the data layer can be switched between two opposing states by an applied magnetic field and thus binary information can be stored. The reference layer usually is composed of a magnetic material in which the magnetization is pinned so that the magnetic field that is applied to the data layer and in part penetrates the reference layer, is of insufficient strength to switch the magnetization in the reference layer.
For example in a TMR cell the data layer and the reference layer are separated by a thin dielectric layer which is arranged so that a tunnelling junction is formed. Any material comprises two types of electrons which have spin-up and spin down polarity. In the case of a ferromagnetic layer that has a magnetization, more electron spins have one orientation compared with the other one which gives rise to the magnetization. The electrical resistance through the layers is dependent on the relative orientations of the magnetizations in the data and reference layers. This is the tunneling magneto-resistance (TMR) effect and the state of the data layer can be read by measuring the apparent electric resistance across the layers.
The data layer typically includes a low coercivity material that can be switched in its magnetic direction by column and row data-write currents.
The reference layer usually is fabricated with a high coercitivity material and is permanently magnetized in a set direction during an annealing processing step.
In one version of the memory cell, namely the so-called the “spin-valve”, the reference layer is “pinned” by exchange coupling by an adjacent antiferromagnetic layer.
Alternatively, the reference layer may be soft-magnetic reference layer and may have a lower coercivity so that the reference layer may be switched together with the data layer. In this case the magnetic field of a control current is used to switch the magnetization of the reference layer to the reference state after the data layer is switched. The coercivity of the reference layer and the magnitude of the control current need to be chosen so that switching the reference layer does not affect the data layer.
In general, the magnetic memory cells should be as small as possible. However, the smaller the cells are made, the more sensitive they are to thermal stability problems during operation. In order to compensate, the small magnetic memory cell data layer are fabricated with magnetic material that is more resistant to magnetic change. Unfortunately, generating the stronger fields necessary makes switching the memory cells more difficult during the write operation. Hence, there is a need for a magnetic memory device in which writing is facilitated.